The present invention relates to the generation of high power electromagnetic radiation in the millimeter and submillimeter wave regime, and more particularly to quasi-optical gyrotron and gyroklystron operation at harmonics of the cyclotron frequency.
The major device currently available for generating millimeter and submillimeter wavelengths is the gyrotron. The gyrotron is a new type of microwave device employing the electron cyclotron maser mechanism. It ideally consists of an ensemble of monoenergetic electrons following helical trajectories around the lines of an axial magnetic field inside a fast wave structure such as a metallic tube or waveguide. The physical mechanism responsible for the radiation in the gyrotron has its origin in a relativistic effect. Initially, the phases of the electrons in their cyclotron orbits are random, but phase bunching (relativistic azimuthal bunching) can occur because of the dependence of the electron cyclotron frequency on the relativistic electron mass (.OMEGA..sub.c =eB/.gamma.mc). Those electrons that lose energy to the wave become lighter, rotate faster, and hence, accumulate phase lead, while those electrons that gain energy from the wave become heavier, rotate slower, and accumulate phase lag. This rotating electron interaction with the wave results in phase bunching such that the electrons radiate coherently and amplify the wave. Energy transfer from the electrons to the wave is optimized when .omega.-k.sub.z v.sub.zo -n.OMEGA..sub.c .gtoreq.0, where .omega.,k.sub.z, v.sub.zo,n, and .OMEGA..sub.c, are, respectively, the wave frequency, axial wave number, axial electron velocity, cyclotron harmonic number, and electron cyclotron frequency.
In essence, there is an intrinsic preference for relativistic azimuthal phase bunching in the presence of an electromagnetic wave. This bunching yields a different configuration of electrons in a lower energy state. If the incident wave has a frequency slightly larger than .OMEGA..sub.c or its harmonics, then stimulated emission will occur. Since this bunching mechanism occurs in phase with the electromagnetic wave, the stimulated radiation emission from the bunching is also emitted in phase with the wave, leading to wave amplification.
The gyrotron stimulated radiation emission occurs near the frequency .omega.=.OMEGA..sub.c +k.sub.z v.sub.zo. Since .omega..sub.c =e.beta./.gamma.mc, the radiation wavelength is determined primarily by the strength of the applied magnetic field and is not restricted necessarily by the dimensions of a resonant structure. Thus, unlike most other microwave tubes, the internal dimensions of the device may be large compared to the wavelength, and high power handling capability becomes compatible with operation at millimeter and submillimeter wavelengths. This high power operation of the gyrotron has been demonstrated.
It is generally desired to scale the cavity gyrotron in accordance with the wavelength such that two or three wavelengths can be set up across the cavity. However, as the wavelength decreases, and the cavity shrinks down, the power density increases and the wall loses become important. Thus, although the operating power level of the gyrotron is high, it is limited by the relatively small interaction volume as the wavelength decreases (the frequency increases). In order to circumvent this limitation, a quasi-optical single or double cavity configuration is utilized. An example of such a quasi-optical design is disclosed in U.S. patent application Ser. No. 414,129, filed on Sept. 2, 1982, now U.S. Pat. No. 4,491,765 by the Inventors, Manheimer, Bondeson, and Ott (Navy Case No. 66,517).
In order to achieve still higher frequency operation and/or operate a reduced magnetic field in the device, it is desirable to operate at harmonics of the cyclotron frequency. A variety of studies (see the citations in the paper "Cavity Design for Quasi-Optical Gyrotron and Gyroklystron Operation at Harmonics of the Cyclatron Frequency" by Levush and Manheimer, International Journal of Infrared and Millimeter Waves, November 1983, pages 877-889) have demonstrated that such harmonic operation may be possible at some reduction in efficiency and/or some increase in wave electric field amplitude in the cavity. However, the difficulty with all of these studies is that they assume only a single cyclotron harmonic is present in the cavity. In actuality, if the third cyclotron harmonic is desired, some means must be utilized to suppress the fundamental and the second harmonic, which are generally stronger processes with much more energy. Without some sort of suppression, the fundamental and the second harmonic would swamp higher harmonics, thereby preventing the generation of high power coherent radiation at such higher harmonics in a controlled manner. In previous studies of harmonic operation of gyrotrons in conventional non-optical cavities, this problem was solved simply by designing the cavity shape so that the fundamental and any other undesired lower harmonics are not eigenfunctions of the cavity. Typically, the cavity is shaped so that it is resonant at only one cyclotron harmonic. (See K. R. Chu, Phys. Fluids, 21, 2354 (1978)) and Zepalof, Korablev, and Tsimring, Radiotech. Electron, 22, 86 (1977). Thus, to operate at the third harmonic, one picks a cavity shape and a mode such that the fundamental and the second harmonic are not resonant.
However, in a quasi-optical configuration, this cavity shaping is not possible because all harmonics are simultaneously resonant in such an optical cavity. In particular, all resonant modes in such an optical cavity are integrally related.